Conventionally, to improve the efficiency in formatting an optical disc, the wobble groove containing clock information and address information has been used. The optical disc according to this technology has an information track on which data is to be recorded, the information track being formed by causing the grooves formed on the optical disc to wobble using a signal that is generated by modifying the rotation control clock by the address information. The technology provides an advantageous effect of increasing the ratio of recording user data with the same recording density since it enables data to be recorded on the entire track without forming an address area. As this technology, the ATIP method for CD-R and the ADIP method for MD are known, for example. The wobble signal can be extracted from the push-pull signal in the tracking-on state. The band of the wobble signal is set to be lower than the band of data modulation signal so as not to affect the data, and is set to be higher than the band of tracking control so as not to affect the tracking.
Meanwhile, it is a major challenge in designing an optical disc apparatus to speed up and stabilize the activation of the optical disc apparatus and the seek, and stabilization of the tracking lead-in during the activation or seek is crucial to overcoming of the challenge. For the stabilization of the tracking lead-in, judgment of the tracking polarity and detection of the moving direction of the optical spot relative to tracks are important. A conventional method for achieving this in ROM discs is to use data envelope on the track. However, this method does not function in recordable optical discs (including a write-once-read-many type and a rewritable type) since they have a non-recording area (that is to say, an area in which no information has been recorded). Japanese Laid-Open Patent Application No. 6-301988 discloses the first method applicable to recordable optical discs, the first method using the reflectance ratio difference between grooves and lands (areas between grooves). With this method, it is possible to intentionally design and form grooves having a reflectance ratio difference by shifting the ratio of land to groove (L/G ratio) from “1”.
In recent development of optical discs using the blue-violet laser, not only the use of wobble grooves but a narrower track pitch is required for achieving higher density. This requirement makes it difficult to secure the reflectance ratio difference between lands and grooves because there is no other choice but to set the L/G ratio to close to “1” since priority is given to securing the formability of optical disc substrate, the wobble signal, the tracking error signal (hereinafter also referred to as TE signal), the recording/reproduction property and the like. Japanese Laid-Open Patent Application No. 2000-3525 discloses the second technology designed to overcome the above problem of the first technology. The second technology is a three-beam system in which two push-pull signals are obtained respectively from two sub-beams, which are shifted from a main beam outward and inward by ¼ track respectively, and a cross track signal is obtained based on a differential between the two push-pull signals, and the obtained cross track signal is used in judgment of the tracking polarity and detection of the optical spot moving direction. This proposal overcomes a problem of a conventional DPP (Differential Push-pull) of being unable to judge the tracking polarity, the problem occurring because the two sub-beams are respectively shifted from the main beam in different directions by ½ track pitch.
Also, Japanese Laid-Open Patent Application No. 2001-202635 discloses the third technology for optical disc tracking control apparatuses for optical discs with wobble grooves. This technology proposes to use a moving direction, which is detected by generating the cross tracking signal on the assumption that “there is, without fail, a difference between lands and grooves in the wobble signal amplitude” in the tracking-off state.
The three-beam system as in the second technology, however, requires a larger laser emission power than the one-beam system. Therefore, for mobile uses that require a small head for the purpose of restricting the power consumption and amount of heat, the one-beam system is suited. If the three-beam system is to be used, a conventional DPP is preferable because it has an advantageous effect of canceling the tracking offset when the data is recorded into the non-recording area.
Also, the assumption that “there is, without fail, a difference between lands and grooves in the wobble signal amplitude” on which the third technology is based cannot be used as a basis for CLV-type optical discs in which the wobble length is basically constant. Similarly, the assumption cannot be used as a basis for DVD-R, DVD+R, DVD-RW, and DVD+RW. This is because in these types of optical discs, the wobble grooves are in a spiral that starts with the inner circumference. The track length per rotation becomes longer as the track is closer to the outer circumference, and the wobble phase is delayed as much if the wobble length is constant. It is considered that in general, the wobble signal amplitude of a land is determined by how wobble signals of two grooves sandwiching the land overlap each other. If the wobble phase is delayed as stated above, the wobble phases of the wobble signals of two grooves sandwiching the land also change. The wobble signal amplitude becomes smaller when the wobble signals are in a reversed phase, but becomes almost equal to the wobble signal amplitude on the grooves when they are in phase. The following will explain this using a specific example. A quantitative explanation will be given later with reference to FIGS. 8-10 when an embodiment of the present invention is described.
FIG. 1 shows a specific example of the case where the above-mentioned assumption cannot be used as a basis, and shows the waveform of a push-pull signal in relation to an optical disc that was manufactured by way of trial by setting the wobble length to be constant (only in regards with clock information). The waveform on the left-hand side is a push-pull signal waveform for one rotation of an eccentric disc in the tracking-off state. The waveforms on the right-hand side are respectively magnifications at (a), (b), and (c) of the waveform on the left-hand side. The figure shows how the wobble signal is superimposed on the TE signal. The waveforms at (a) and (b) are those of approximately the same track and have opposite track moving directions relative to the optical spot. In the waveforms at (a) and (b), the wobble signal amplitude is decreased at decay and rise, respectively.
In the above waveforms, the portions having small wobble signal amplitudes can be judged as lands. The moving direction was confirmed by comparing the jumping waveforms which were observed separately from the waveforms. According to the observation, the waveforms at (a) and (b) have optical spot moving directions respectively outward and inward relative to the track. However, in regards with the wave form at (c), the moving direction cannot be determined from the wobble signal amplitude because the wobble signal amplitude is constant at both rise and decay.
The specific data provided above indicates that in the CLV-type optical discs, the assumption that “there is, without fail, a difference between lands and grooves in the wobble signal amplitude” cannot be used as a basis. Also, in optical discs in which the wobble signal is modified by the address information practically, there is a problem that even a portion, where it is expected that the wobble phase is reversed on lands to decrease the wobble signal amplitude, becomes partially in-phase (or close to in-phase) due to the modulation and thereby generates a spike-like increase of the wobble signal amplitude, though the problem depends on the modulation method. That is to say, the third technology has a problem that the wobble signal amplitude is not enough to judge the tracking polarity or detect the optical spot moving direction.
Meanwhile, the assumption that “there is, without fail, a difference between lands and grooves in the wobble signal amplitude” cannot be used as a basis even for ZCLV optical discs such as DVD-RAM. More specifically, in DVD-RAM, the wobble is used only in the clock signal. In DVD-RAM, the groove wobble is formed so as to be in-phase within a zone so that a wobble signal amplitude equal to the groove can be obtained in recording on lands, as well as in recording on grooves. That is to say, even on a land between grooves, the wobble signal amplitude is not decreased. It should be noted here that mediums such as DVD-RAM in which the grooves and lands have the same wobble signal amplitude are not the object of the present invention.